Dear Ahmadreza Shafieyan ;

you can try Barium-Dispenser Thermionic Cathode, as detailed in the references;

https://worldwidescience.org/topicpages/b/barium+dispenser+cathode.html

Dear Ahmadreza Shafieyan,

Metal sputter coated dispenser cathodes or thermionic dispenser cathodes are generally emits electrons through couple of metal thermionic emission; moreover used in microwave, telecommunication devices and sometime critical space components. Barium dispenser cathodes are quite popular. noble metals such as Os, Ir, and Re on the surface of tungsten dispenser cathodes enhances the emission capabilities and uniformity.

Ashish

Hi. I hope the following thesis could help you:

https://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1040&context=cme_etds

Hi. I think electroplating technique is the most cheapest methode in making a coat of elements on cathides.